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(*
Title: Standard Basis Library: Option Structure
Author: David Matthews
Copyright David Matthews 1999, 2005
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*)
(* G&R 2004 status: updated, added "app" function. *)
signature OPTION =
sig
datatype 'a option = NONE | SOME of 'a
exception Option
val getOpt : ('a option * 'a) -> 'a
val isSome : 'a option -> bool
val valOf : 'a option -> 'a
val filter : ('a -> bool) -> 'a -> 'a option
val join : 'a option option -> 'a option
val app : ('a -> unit) -> 'a option -> unit
val map : ('a -> 'b) -> 'a option -> 'b option
val mapPartial : ('a -> 'b option) -> 'a option -> 'b option
val compose : (('a -> 'b) * ('c -> 'a option)) -> 'c -> 'b option
val composePartial : (('a -> 'b option) * ('c -> 'a option)) -> 'c -> 'b option
end;
structure Option : OPTION =
struct
(* datatype 'a option = NONE | SOME of 'a *)
datatype option = datatype option
exception Option
fun getOpt (SOME a, _) = a
| getOpt (NONE, b) = b
fun isSome (SOME _) = true
| isSome (NONE) = false
fun valOf (SOME a) = a
| valOf (NONE) = raise Option
fun filter f a = if f a then SOME a else NONE
fun join (SOME v) = v
| join NONE = NONE
fun app f (SOME v) = f v
| app _ NONE = ()
fun map f (SOME v) = SOME (f v)
| map _ NONE = NONE
fun mapPartial f (SOME v) = f v
| mapPartial _ NONE = NONE
fun compose (f, g) a =
case g a of
NONE => NONE
| SOME v => SOME (f v)
fun composePartial (f, g) a =
case g a of
NONE => NONE
| SOME v => f v
end;
(* Export some of these to the top-level. *)
local
structure dummy:
sig
(* datatype 'a option = NONE | SOME of 'a *)
exception Option
val getOpt : ('a option * 'a) -> 'a
val isSome : 'a option -> bool
val valOf : 'a option -> 'a
end = Option
in
open dummy
end;
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